Transmission through space discharge device



w. e. SHEPHERD TRANSMISSION THROUGH SPACEDISCHARGE DEVICE Filed May 17,1940 F/Gf/ 0. p I2-3 4 s 6 7 9 a m EZECTROA/[A/ERGY 2 /v 011v erg/aw y[MM 11 .0%.

FIG. 3

CURRENT POTENTIAL or CHAMBERS RELATIVE TO GRID 4 I) LOAD l3 /4 I 2Sheets-Sheet l F/GZ 24 5 6 GAS FILLED /o l l l0 Hlkg /? INDICATOR INVE/V70/? W G. SHEPHERD ATTORNEY June 30,1942. w. e. SHEPHERD 2,288,256

TRANSMISSION THROUGH SPACE DISCHARGE DEVICE Filed May 17, 1940 2Shets-Sheet '2 ENERGY 11v ELFCTRON VOL rs FIG 6 IN VE N 70/? n: a.SHEPHERD ATTORNEY Patented June 30, 1942 TRANSMISSION THROUGH SPACEDISCHARGE DEVICE William G. Shepherd, Bayside, N. Y., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application May 17, 1940, Serial No. 335,647

20 Claims.

The present invention relates to space discharge apparatus and circuitstherefor. More specifically, the invention relates to apparatusutilizing electron discharge through gas, in which certain efiects ofelectron scattering or effects of collisions of electrons with gasparticles are made use of for such purposes as securing amplification ofinput waves, producing a negative resistance, or controlling the currentin an eX- ternal circuit for any purpose.

When a stream of electrons traverses a gas filled region some of theelectrons will undergo collisions which will, in general, change boththe magnitude and direction of their velocities.

By separating the electrons which have undergone collisions from thosewhich have not, it is possible to derive an outputcurrent whosemagnitude depends upon the rate of collisions. By controlling somequantity which determines the rate at which collisions occur, it is thenpossible to vary the output current.

The invention provides method and means for accomplishing these results.Electrons are projected into a region containing gas under conditionsproducing collisions. Control of the rate of collisions is obtained bycontrolling the velocity at which electrons are projected into thisregion; and the electrons which have not undergone collision arecollected as Outputcurrent, while the electrons that have experiencedcollisions are diverted from the collector in a manner to be described.7

As stated, the velocity of the electrons that undergo collision ischanged. The electron may be deflected to one side or it may be slowedup with little or no change of direction. Separation of electrons thathave not collided from electrons that have collided can be made on thebasis of their velocity difierences. Two general ways Will be disclosedherein, one depending on angle of deflection and the other depending onloss of velocity in the forward direction.

The nature of the invention and its objects will be more fully set forthin the following detailed description in which will be given the generaltheory together with certain preferred ways of carrying out theinvention in practice. These illustrative embodiments are shown in theaccompanying drawings, in which:

Fig. 1 shows curves to be referred to in the description of the theoryand operation;

Fig. 2 is a diagrammatic representation of one type of tube which may beused in accordance with the invention to secure novel results;

Fig. 3 shows curves illustrating the operation of the tube of Fig. 2;

Fig. 4 showsa sectional view in elevation, and circuit diagram, of onetype of tube according to the invention;

Fig. 5 shows a perspective view, partly broken away, of the tube of Fig.4;

Fig. 6 is a schematic representation of an alternative type of tube andcircuit according to the invention;

Fig. '7 shows transconductance curves such as are obtainable inaccordance with the invention;

and

Figs. 8 and 9 show tubes and circuits similar to Figs. 4 and 6 butwith'push-pull outputs.

In the following discussion of the theory and operation the generalassumption is made that the electron energies are kept below theionization value. This simplifies the treatment and, moreover, accordingto applicants. present information, this represents the preferredpractice as will be indicated later on. This is not known, however, toconstitute a limiting condition for the invention and there is no intentto limit the invention to the region below the ionization level butrather that the invention shall extend to all regions and fields inwhich it may be applicable in practice.

The probability that an electron in traveling unit distance understandard conditions of temperature and pressure will undergo a collisionmay be represented by P. The meaning of the probability is that if astream of electrons of current strength I passes a distance dzc throughH the gas and if the current measuring device at the end of the gaschamber will accept and indicate only those electrons whose Velocitydoes not differ in magnitude or direction from that of the electronsentering the gas chamber by more than some definite increment, then thedecrease in the current strength will be -dI=IPpdr (l) The factor p, thepressure, appears since the chance of collision will increase in directproportion to the number of molecules present,

The final current received at the collector is found by integrating (1)to give I =I0e $17 (2) which may be written in a slightly different form-Nnap1: I =I e (3) where No is the number of molecules of gas per unitvolume at pressure 120 and a has the dimensions of an area and is calledthe cross-section for collision of an electron with the molecule.Further reference to Formula 3 will be made later on.

From (2) it is seen that the output current in the case of a fixed pathand gas pressure depends only on the collision probability or rate.Attention will be confined for the moment to elastic collisions and somediscussion will be given later of the effect of inelastic collisions. Inan elastic collision an electron loses only that amount of energynecessary for the conservation of momentum in the collision. If theelastic collisions between the electrons and gas molecules were exactlysimilar to those between hard spheres then the probability or rate ofelastic collision would be independent of electron energy and thereforeindependent of the electron velocity. The electron has, however, waveproperties and the wave-length associated with the electron depends uponits energy. As is the case with the scattering of light from particlesthe scattering will then be a function of the wave-length or energy ofthe electrons. be the correct and complete theory to account for it, theoutput current can be controlled by varying the velocity with which theelectrons are projected. into the collision space. The curves given inFig. 1 are taken from a paper by Ramsauer published in PhysikalischeZeitschrift 1928, vol. 29, page 823, and show the probability ofcollision of electrons in argon, krypton and xenon as a function of thesquare root of their energy in electron volts which is proportional totheir velocity. Similar variations for the probability of collision andelectron scattering are exhibited by other gases.

R. Kollath and E. Steudel in studying electron scattering used a tubestructure of the general type shown in Fig. 2. Their work is publishedin Zeitschrift fur Tech. Physik 1939, Nr. 2, pp. 36-38. In this figurethe tube l contains a cathode 2, grid electrode 4, scattering chamberand collector 6. tentials to the electrodes 4, 5 and 6. An indicatinginstrument II] shows the output current. A source of variable voltage Il is provided for varying the potential of the chamber 5 and the currentin this connection is indicated at l2. electron stream enters thechamber 5 through the slit opening shown and, depending upon themagnitudes of the voltages applied to electrodes 4 and 5, passes throughto the collector 6, which has positive voltage applied to it by 9sufficient to insure that all the electrons leaving 5 will be collectedon 6. A by-pass condenser is shown at 3.

Assuming first that the tube contains no gas, if the current at In isobserved as the voltage of source H is varied starting from negativevalues greater than cut-off and progressing through zero to positivevalues, curve a of Fig. 3 is obtained. If some suitable gas such asargon is admitted at a suitable pressure related to the length of thescattering chamber, e. g. a pressure of 5 10 millimeters Hg for a pathlength of 1 centimeter, and if the current at I9 is again observed as afunction of the voltage of source II, the curve a will be retraced inpart but will then take the course indicated at b. A xenon filling atthe same pressure would call for a path length of .50 centimeter. If, asassumed, there is no At any rate, whatever may Batteries 1 and9 applypositive po- The i ionization, no new ions are formed and the totalcurrent will remain the same. The current I12 to electrode 5, observedat I2, is therefore given by the difference between curves a and b, orcurve 0 in the figure. This curve 0 on the righthand side of its maximumpoint exhibits a decreasing current I12 for increasing applied controlvoltage, or a negative resistance effect in the circuit to the chamber5.

The current I12 is the current that is diverted to the chamber 5 byelectron scattering. Electrons suffering more than a certain smallangular deflection fail to pass through the output aperture towardcollector 6 but are collected on the walls of chamber 5. This device,therefore, makes use of the angle of deflection to separate the currentinto two components.

In accordance with the invention this negative resistance effect can beutilized, as by biasing the chamber 5 to some suitable point k by meansof source H and applying a variable or input voltage at [3 from a source[5 in any suitable manner to operate the tube about point k: on thefalling portion of curve 0. The negative resistance efiect is producedin the circuit carrying current I12 and may be utilized in a suitableload impedance related to this circuit. For example, I2 may in this casebe the load or the load may be in a circuit such as 14 coupled to theI12 branch, for example, it may be in series with the source of controlvoltage or input voltage I5.

Thiscircuit may be used for amplification between input variations at [3and output variations at It! in similar manner to the tube de scribedbelow in connection with Figs. 4 and 5.

It is important in the operation of this device that the strength of theelectron current entering the scattering chamber shall not change as afunction of the voltage variations applied to the chamber but that onlythe electron velocity shall change. The grid 4 is arranged to draw outthe electrons from the cathode and provide a constant supply to thechamber. The potential difference between the cathode and grid 4 isconstant. The velocity of the electrons projected into the chamber 5 iskept below the ionization level.

Another form of device depending upon angle of deflection to separatethe total current into two components is shown in Figs. 4 and 5. This isa cylindrical type tube with cathode 20, accelerating grid 2|, positivegrid 22, scattering chamber 23, shield grid 24 and plate 25. Cells 26bias grid 2|. The control potentials or input variations (signal) areapplied between chamber 23 and cathode from a suitable signal or otherinput circuit 21 through transformer 28. Grids 22 and 24 are shownstrapped directly to chamber 23 and these three elements are given abias of suitable type from source 29 of sufiicient range ofadjustability to permit of a positive or negative bias relative to grid2|. Cells 30 place positive potential on the anode or collector 25 andoutput transformer 3! leads to outgoing utilization circuit 32.

Collisions occurring in chamber 23 produce electron scattering and thosescattered electrons that have a sufliciently large angle of deflectionare collected on the walls of the compartments into which the scatteringchamber 23 is subdivided as indicated. The electron current entering thechamber 23 is kept constant and of the desired value by the first grid2| which should be of sufficiently fine mesh to prevent changes in thefields from the outer electrodes from appreciably affecting the currentdrawn from the cathode. The electron current is, thus, as a result ofthe collisions and scattering, resolved into a component passing to theelectrode 23 and another component to the anode 25. Grids 22 and 24serve to keep the chamber 23 as a whole immune from the influence of therelatively variable fields existing on each side of it. These grids,especially grid 22, may not be necessary in actual practice. Thepositive potential on the plate 25 is of such value as to bring thevelocities of the electrons emerging from the chamber 23 up to a usefulvalue. Alternative connections from the negative pole of battery 30 topoint 33 or point 34 are indicated, and can be effected by a switch, ifdesired. Connection to point 33 returns the alternating current from thechamber 23 through the secondary of input transformer 28. This may beavoided and instead the results in decreased output for a positive swing77 of the input or control voltage and increased output for a negativeswing of the control voltage. This means that the tube has a negativetransconductance from the control voltage to the output current whenoperated within the lm part of the characteristic. The m-n regioncorresponds to positive transoonductance since a positive swing in inputvoltage produces increasing output current and vice versa. These curvesapply to the tubes of Figs. 4 and 5 as well as to the tube of Fig. 2.

Referring to Fig. 4, adjustment of the slider on resistor 29 determinesthe portion of the tube characteristic on which the tube is operated.

Movement of the slider to the left in the figure it (or similar movementof the slider on resistance H of Fig. 2) makes the bias less positiveand may be thought of as moving the operating point to the left in Fig.3, for example, to some point intermediate between Z and m. A morepositive bias will shift the operating point to the right, for example,to the region mn.

The negative resistance efiect described in connection with Fig. 2 canbe obtained in similar manner in the case of Fig. 4 using the connectionat point 33. The effect appears in series in the lead to the chamber 23.

It was stated above that the preferred voltage operating range is thatbelow the ionization level.

In the case of the particular gases indicated in Fig. 1 the mostadvantageous operating ranges would be either on the left-hand slopes oron the right-hand slopes of the curves since these portions are thesteepest. Another reason for keeping below the ionization potential isin order that positive ions may not be produced in large numbers in thebody of the gas since they would tend to drift back toward the cathodewhere they would produce space charge eiTect and change the supply ofelectrons entering the chamber.

Thi would give variable or unstable operation.

Fig. 6 shows a tube in which the electrons that have lost velocity dueto collisions are stopped by a grid 36, which is preferably atsubstantially cathode potential but may have its potential varied fromcathode potential in either positive or negative direction by movementof slider 39 as may be desired in any particular case. The similarlynumbered parts may be the same as in Fig. 4. The control potentials areapplied between the grid 2| and another suitable electrode and are hereshown as impressed between the positive grid 2! and a grid 35 locatedjust outside grid 21. space between grid 2| and stopping grid 35. Theelectrons that have been slowed down to a velocity below a given valueare unable to traverse the retarding field of grid 36 and are attractedtoward grid 35 while those electrons with greater velocity than thisgiven value are able to pass through to the accelerating field of theanode 38 or of the accelerating grid 3'! if used. Grid 3? is connectedto a point of suitable positive potential in battery 30, such as to itspositive terminal. By throwing switch 4| to connect to point 42 or 43either the alternating current from grid 35 or the direct current frombattery 30 is returned through the input coil as explained above underFig. 4.

In the Fig. 6 construction as in the case of Fig. 4, the grid 2| is usedto flx the current so that variations in potential of the control grid35 will not change the value of the injected current. In this type oftube the electrons need not collide with a wall in order to be removedfrom the current stream, hence the spacing between grids 35 and 36 maybe made as small as mechanically possible. This fact has an im portantbearing on the tube design and performance as may be seen from acomparison with the case depending on angular deflection. In both casesit is essential that the density of the current injected into thescattering space be sufliciently low so that a virtual cathode will notbe formed for the lowest velocities reached by the injected electrons,since the action of the virtual cathode is partially to compensate theaction of the collisions. The permissible current density depends onlyupon the path length in the scattering space and the lowest energy ofthe electrons entering this space.

Since the length of the scattering space can be so much smaller in thestopping grid case, for the reason given above, the density of theinjected current can be greater. The permissible injected currentdensity varies inversely as the square of the length of the scatteringspace, which means that the current density may be increased in theorder of twenty-five fold over the optimum angular discrimination casewith a corresponding increase in the figure of merit.

It should be noted that the stopping grid method discriminates againstall the electrons that have undergone collisions, while in the methoddepending on angular deflection, only those electrons experiencing morethan a given angular deflection are removed from the electron stream.Electrons deflected from their initial direction, even if they shouldmaintain the same numerical value of velocity, lose velocity in theforward direction and are stopped by the stopping grid. Moreover, thestopping grid method is particularly effective in the case of collisionsinvolving excitation since in this case the velocity changes would belarge.

Up to this point only the elastic type collisions The useful collisionsoccur in the this number will vary with variations in velocity of theinjected current independently of the variation in number of elasticcollisions. The total collision rate will be the sum of the individualrates. Collisions resulting in excitation require that a definitequantum of energy be given up by the electron. Hence the probability P5,meaning that a certain fraction of the electrons in traveling unitdistance under given conditions of temperature and pressure will undergocollision producing excitation, will have a definite energy threshold. Acontrol of the electron velocity will, therefore, give a control of P5and thus a control of the transmitted current. The total probability ofelectron collision (the subscript e referring to the elastic collisioncase) and the total current 1:: due to both types of collisions is givenby 1 10 61:27 [*(Ps-i-Pe) 1383] which is similar to the expression inEquation 2. In both the types of discrimination depending upon angulardeflection and stopping grid action, the transmitted current is given by3. this formula the probability of collision P0 is a function of theelectron energy which will depend upon the potential drop through whichthe electrons falls prior to the scattering chamher. Since No and 210are fixed 0' is a function of this potential and hence thetransconductance,

deg p0 beg N pa: at 1 p0 Thus, if the operating point is in a region inwhich the cross-section for collision increases with energy a negativetransconductance can be obtained and a positive transconductance wherethe operating point is in a region where the cross-section decreaseswith energy.

The transconductance is a function of pressure and an investigation ofthe optimum pressure condition shows that for a maximum N pax which maybe interpreted as a condition on 1 :0.

The transmitted current under these conditions is 1:106 and max l 2i 0'56,;

Thus for a maximum transconductance i n 0' be; should be maximum. Usingthe experimental values of 0' versus electron energy given in Fig. 1

X l0 ,umhos/milliampere data for gm versus energy are shown for argonFig. 4 is shown but in this case the output circuit is connected inpush-pull relation to the anode 25 and scattering chamber 23, thusutilizing both of these elements as output electrodes. Balancedtransformer 43 couples these electrodes to load circuit 32 in push-pull.anode more positive than the other tube elements. Since the electroncurrent entering the chamber is substantially constant in density and isdivided into two components in complementary manner, substantially equaland opposite variations occur in the current to the chamber 23 and theanode 25. With no signal impressed at 2T, 28 the entering electroncurrent into the chamber divides in some proportion, depending on thebias potential, into a steady component to the anode 25 and anothersteady component to the chamber 23. An input signal variation which isin a direction to increase the probability of scattering decreases thecurrent to the anode and increases the current to the chamber, and viceversa. Transformer 40 is so wound as to make these current changescumulative in their effect on the output 32.

Fig. 9 shows how a push-pull output may be applied to a tube using thestopping grid method. The divided primary of transformer 40 has itsoutside terminals connected across the anode 38 and control grid 35. Inthis case, in the absence of any signal input, the electron currentprojected into the space between grids 35 and 36, assuming a bias nearthe middle of some operating range, passes in part to the anode and isin part stopped at 36, the portion stopped con sisting of slowedelectrons that are unable to pass through the retarding field of grid36. These electrons drift back to the positive grid 35 and are returnedto the cathode through the lower primary winding of the transformer 40.When signal variations are present in 21 the portions of the totalcurrent passing to the anode and to the positive grid 35 vary oppositelyand substantially equally and the variations to each electrode actcumulatively in the output circuit 32.

In both Figs. 8 and 9 feedback occurs from the output to the input.Assuming a positive transconductance, an increase in voltage of 23 or 35in the positive direction increases the current to the anode anddecreases the current flowing in the circuit of 23 or 35. This resultsin decreased drop of potential across the lower half of winding All]which is in a direction to augment the assumed initial efiect. Hencethis feedback is positive. With negative transconductance negativefeedback is obtained.

The push-pull connection reduces even order distortion. Positivefeedback increases the gain. Negative feedback reduces distortion,improves the gain stability and is accompanied by the usual advantagessecured by negative feedback in amplifiers generally.

The invention is not to be construed as limited to the particularconstructions of tube or circuit that have been disclosed since theseare to be considered as illustrating the principle and manner ofoperation of the invention, the scope of which is defined in thefollowing claims.

What is claimed is:

1. The method comprising controlling the current between a first and asecond electrode in a tube containing gas by establishing current flowbetween said electrodes and diverting current away from said secondelectrode by pro- Battery 64 makes the duction of electron scattering ina region between said electrodes by collision between electrons and gasmolecules, variably controlling the strength of the current diverted bysuch scattering, and utilizing the current so diverted.

2. The method according to claim 1 in which the component of theelectron velocity in the direction toward said second electrode isdependent upon the scattering, including the step of interceptingelectrons having less than a predetermined magnitude of velocitycomponent in said direction.

3. The method of controlling transmission through a discharge devicecontaining gas comprising producing electron scattering therein bycollision between electrons and gas molecules, stopping transmission tothe outputof the device of electrons that have lost velocity because ofsuch scattering and producing flow of current in said output from theremaining, fastmoving electrons.

4. The method of producing output current under control of input voltagevariations comprising producing electron discharge between electrodes ina gas, producing electron scattering by collision between electrons andgas molecules, variably controlling by the input voltage variations theelectron velocity, such variations being confined to a region withinwhich the collision probability varies markedly and in the same sensewith variations in electron velocity, separating the scattered electronsas one group from the electrons that have not experienced scattering asanother group and producing output current flow from the electrons ofone of said groups.

5. The method of producing output current under control of input voltagevariations comprising producing electron discharge between electrodes ina gas, producing electron scattering by collision between electrons andgas molecules, the scattering electrons experiencing reduced velocity inthe forward direction, variably controlling by the input voltage theelectron velocity between limits within a region in which the collisionprobability varies markedly with variations in electron velocity,stopping the electrons with reduced velocity in the forward directionresulting from the scattering while allowing the fast-moving electronsto proceed thereby separating the reduced velocity electrons and thefast-moving electrons into groups and producing output current flow fromthe electrons of one of said groups.

6. The method according to claim 4 in which the scattered electronsexperience angular deflection with respect to their original directionand in which said separating comprises intercepting electrons havinggreater than a predetermined angle of deflection with respect to theiroriginal direction of travel as a result of such scattering whileallowing electrons of a lesser angle of deflection to proceed.

7. The method of producing output current variations under control ofinput voltage variations comprising producing electron discharge betweenelectrodes in a gas, varying the probability of collision betweenelectrons and gas molecules by varying as a function of the inputvoltage variations the electron velocity between limits within a regionin which the collision probability varies markedly with variations inelectron elocity and producing output current variations of a magnitudedependent upon such variations in probability of collision.

8. The method of according to claim '7 in which the variation ofprobability of collision when plotted with respect to electron velocityfollows a characteristic curve of varying slope, the method includingcausing such variations in input voltage to take place about anoperating point near the middle of a steep portion of the characteristiccurve between electron velocity and probability of collisions.

9. A space discharge device having a centrally positioned cathode andconcentrically arranged electrodes surrounding said cathode in anenvelope containing gas, means producing discharge between saidelectrodes, means producing scattering of electrons within said envelopeby collisions between electrons and gas molecules, means for variablycontrolling the amount of electron scattering comprising meansresponding to an input voltage for varying the electron velocity, andmeans deriving an output current from said device of a magnitudedepending upon the amount of such scattering.

10. A space discharge device comprising a cylindrical envelopecontaining gas, a centrally located source of electrons therein, meanscooperating with said electron source for producing a stream ofelectrons in said gas of substantially constant current strength, asource of input voltage variations, an output electrode concentric withrespect to said source and means controlled by said voltage variationsfor varying the velocity of said electrons, thereby to vary the rate ofcollision between said electrons and gas molecules, said means forvarying the velocity of said electrons comprising means for divertingelectrons from said stream at a point between said electron source andsaid output electrode, at a rate dependent upon said rate of collision.

11. A device according to claim 10 in which the diverted electrons areangularly deflected by different amounts as a result of variations ofelectron velocity and in which said means for diverting electronscomprises an annular scattering chamber having a plurality of orificesof restricted dimensions on the side toward said output electrode andwalls for intercepting electrons having too great an angle of deflectionto pass through said orifice.

12. A device according to claim 10 in which said means for divertingelectrons comprises a stopping grid interposed ahead of said outputelectrode and means maintaining said grid at low potential relative tosaid output electrode.

13. In combination, a space discharge device comprising an envelopecontaining gas, cathode and. anode electrodes therefor, means producingelectron discharge between said electrodes, means producing scatteringof electrons within said envelope by collisions between electrons andgas molecules, a control electrode located between said anode andcathode, a source of voltage variations connected between said cathodeand said control electrode for varying the velocity of electrons therebyto control the amount of scattering, means to bias said controlelectrode to produce a negative resistance effect in the circuit betweensaid cathode and control electrode, and means to utilize said negativeresistance effect.

14. In combination, a space discharge device comprising cathode andanode electrodes in an envelope containing gas, means producing electrondischarge between said electrodes, means producing scattering ofelectrons within said envelope by collisions between electrons and gasmolecules, a control electrode located between said cathode and anode, asource of variable voltage connected to said control electrode forvarying the electron velocity and, as a function thereof, the amount ofsuch scattering, said control electrode constituting a collector ofscattered electrons, and means in circuit with said cathode and controlelectrode for utilizing the current resulting from electrons collectedby said control electrode.

15. In a space discharge device, an envelope containing gas, a cathodeand, in the order named, a first grid, a second grid, a third grid andan output electrode, means to apply positive potential to the first andsecond grids, a low potential including zero value to the third grid andpositive potential to the output electrode, means including saidelectrodes and potentials for producing electron discharge in saiddevice and scattering of electrons by collision between electrons andgas molecules, means including a source of variable potential applied tosaid second grid for varying the amount of such scattering, therebyvariably diverting electrons away from said output electrode, and meansconnected to said output electrode for utilizing the current variationsresulting from such variable diverting of electrons therefrom.

16. The combination according to claim 15 including a push-pull outputconnection between said second grid and said output electrode with itsmid-point connected to said cathode.

17. In combination, a space discharge device containing gas, a source ofsupply of electrons therein of substantially constant density forproducing electron scattering by collision between electrons andmolecules of said gas, a control grid, a stopping grid and an outputelectrode in the order named, means to apply varying po. tential to saidcontrol grid to vary the velocity of electrons in the direction towardthe output electrode, means comprising the amount of gas pressure usedand the velocity imparted to the electrons for operating said tube on asteep portion of the characteristic curve between the rate of electronscattering and the velocity of electrons whereby the variation inscattering is controlled by variations in said potential applied to thecontrol grid and the current to said output electrode is varied inaccordance with the variation in electron scattering, and a circuitconnected to said output electrode for utilizing the resulting varyingoutput current.

18. In combination, a space discharge device containing gas, a cathodetherein, an accelerating grid adjacent the cathode and means forapplying a steady positive potential thereto, a control grid beyond theaccelerating grid, means to apply a variable signal voltage thereto, astopping grid beyond the control grid and means to maintain itspotential at substantially cathode potential, an output electrode beyondsaid stopping grid and means to apply positive potential thereto, meanscomprising the pressure of the gas and the velocity with which theelectrons are injected by said accelerating and control grids into thespace between said control grid and stopping grid for producingeffective electron scattering as a function of said signal voltage,whereby the current to said output electrode is varied in accordancewith variations in electron scattering, and signal responding meansconnected to said output electrode.

19. A space discharge device comprising an envelope containing gas, asource of electrons therein, means cooperating with said electron sourcefor producing a stream of electrons in said gas of substantiallyconstant current strength, a source of input voltage variations, meanscontrolled by said voltage variations for varying the velocity of saidelectrons, thereby to vary the rate of collision between said electronsand gas molecules, an output electrode and output circuit, and meanscomprising a stopping grid interposed ahead of said output electrode fordiverting electrons from said stream at a rate dependent upon said rateof collision.

20. A combination according to claim 19 including feedback means forvarying the velocity of said electrons under control of currentvariations in said output circuit.

WILLIAM G. SHEPHERD.

